As the number of radio communication units in use increases, the potential interference faced by each individual radio communication unit increases. As a result of this increase, there has grown a distinction between two different types of users of the frequency spectra: primary and secondary users. Primary users are licensed users of the frequency bandwidths. Traditionally, these devices are narrow band devices, modulating their transmission signals using a protocol such as Global System for Mobile communication (GSM) or a wide variety of other protocols where each transmission is confined within a narrow bandwidth within the frequency spectrum.
Secondary users follow protocols that ensure that their transmissions do not interfere with primary users. This responsibility of a secondary user to ensure that it does not interfere with primary users is traditionally referred to as the secondary user deferring to primary users of the spectra.
There is no requirement that the primary users refrain from interfering with secondary users. However, secondary users should assume that their transmissions will encounter interference from primary users. Traditionally, this interference has been mitigated once the signal is received at the receiver, through the use of filters. These filters reduce likely interfering signals while having minimal effect on desired signals. This interference, traditionally mitigated at the receiver, can arise from many different sources, including interference from radio communication devices near the transmitter.
Power consumption has also become a driving concern for many users. Handheld devices are becoming smaller with increasing functionality. Contemporary devices not only use battery power to make and receive phone calls, but also to run many different applications, ranging from games to business tools. As the sizes of the devices decrease, the dimensions available to store a battery in the device also decrease, requiring smaller batteries be used while still providing enough power to run the devices for a reasonable period of time.
In the situation of high interference levels, even though the receiver can receive a signal, and remove a large portion of the interference, the integrity of a bidirectional link may be compromised. Although traditional methods increase the likelihood of successful signal reception, the amount of processing and power used is significant.
In spread spectrum protocols, such as Code Division Multiple Access (CDMA), in order to reduce the effects of interference upon a signal, traditionally, a receiving device will pass the signal through a whitener. The whitener estimates the received signal's spectrum and normalizes the signal for further processing. This normalization procedure can be done in many different ways, including comparing the received signal to a reference signal having properties similar to that of the transmitted signal, or normalizing the power level of the received signal. Although this will increase the chance that a transmitted signal is successfully received by the receiver, the transmitter uses its power inefficiently. For example, the transmitter may expend power to transmit in the frequencies that the receiver will not detect due to interference.
The protocol of using a spread spectrum for transmission can avoid significant interference from conventional narrow band sources, as the signal is being spread over a wider bandwidth. This is because the bulk of the signal may still be successfully recovered from frequencies having minimal interference, rather than relying on frequencies that may be too noisy to demodulate. Although this protocol can help tolerate significant interference, the interference-free portion of the spread-spectrum bandwidth decreases with the increase of interfering transmitting devices, making reception more difficult. Traditionally, in order to compensate for this interference, the transmitting device would increase the power used to transmit the signal.
Specifically in the situation of secondary users deferring to primary users, increasing the power not only increases the amount of power inefficiently used in the communication system, it also increases the likelihood that the transmission from the secondary user will interfere with communication by the primary user. As a result, these users traditionally have had to rely upon the ability of whiteners and filters at the receiving device in order to demodulate a signal successfully.
These concerns will only increase as more devices are used, not only in the private sector, but also in governmental, military and other specialized sectors.